Q: Uninstalling an App = devices deleted?

Since I was having ongoing issues with Aqara sensors and a restart app/reboot Homey/PTP did not help, my thought was to try and remove the Mi/Aqara app, reboot Homey and reinstall the app, expecting it to reconnect the orphaned devices again. Unfortunately that was not the case and all linked devices had been removed permanently.

Is that expected behavior? I think on Hubitat I could freely add/change/remove drivers without losing devices?

Correct, uninstalling apps removes devices.
From cli as developer you can install upgrade apps without removing devices.